The present invention relates broadly to a microwave calibration standard apparatus, and in particular to a microwave fiber optic reflection calibration standard apparatus.
The state of the art of testing and utilizing optical fibers is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Patents:
U.S. Pat. No. 3,770,350 issued to Stone et al. on Nov. 6, 1973; PA0 U.S. Pat. No. 4,197,007 issued to Costa et al. on Apr. 8, 1980; PA0 U.S. Pat. No. 4,243,320 issued to Gordon on 6 Jan. 6, 1981; PA0 U.S. Pat. No. 4,391,517 issued to Zucher et al. on July 5, 1983: and PA0 U.S. Pat. No. 4,816,669 issued to Andersen on Mar. 28, 1989.
The Stone et al. patent discloses the use of a long length of liquid-core optical fiber which is employed as a Raman cell. The fiber core contains the material whose Raman spectrum is to be analyzed.
The Costa et al. patent is directed to a device for evaluating the light transmitting characteristics of optical fibers by intercepting and measuring the radiant energy which is back-scattered from the entrance end of an optical fiber irradiated by a pulsed laser beam.
The Gordon patent describes methods for testing optical fibers to determine fiber length and the position of a reflective discontinuity in a fiber without the sophisticated and costly equipment required by current optical time domain reflectometry techniques.
The Zucher et al. patent discusses a method of measuring splice loss in optical fibers by obtaining a reference level of the light that is transmitted out of one end of the input fiber before making the splice. After the splice is made, with light on the reference level in the input fiber being incident on the splice, the radiometer provides an indication of light lost as a result of the splice.
The Andersen patent describes a method for determining the location of a reflection point along the fiber. A process for time localization of reflected pulses is utilized wherein at least two reflected pulses being received, the first one of which originates from a predetermined reference point, the second one from a reflection point whose position is to be determined.
In FIG. 1 there is shown a prior art reflection calibration test configuration. The network analyzer comprises a Hewlett Packard HP8702A lightwave component analyzer. With this test configuration, the network analyzer operates in a forward transmission measurement mode and provides microwave modulation to the laser. The microwave modulated optical signal propagates through the fiber optic directional coupler and experiences a 4 percent reflection at each glass/air interface at the end of fibers #2 and #3. The reflected portion of each signal propagates back through the directional coupler where approximately half of the reflected signals are directed to fiber #4. The signal emerging from fiber #4 is demodulated by a p-i-n photodiode, and the resulting photo-current returns to the network analyzer. One reflection occurs at the end of fiber #2, and the second reflection occurs at the end of fiber #3. This leads to a summation of the optical power of each reflected wave, which depends on the relative microwave phase between the two reflected signals and the coupling symmetry of the directional coupler. The reflection magnitude and phase, measured at fiber #4, is unique to each directional coupler that is manufactured. Each HP8702A unit contains electronic firmware which stores the calibration data of the particular directional coupler that is delivered with a given unit.
The accuracy limitations of this practice arise from two key factors. First, two reflections occur using this technique. As the quality of the cleaved fiber endfaces degrades over time, the reflections become less than 4 percent which leads to errors in magnitude measurements. Further, since the path length of the optical fiber in the directional coupler is temperature-dependent, the actual path length and insertion loss of the directional coupler at the time of measurement can vary significantly from the calibrated data stored within the machine, which leads to errors in phase and magnitude measurements. Secondly, this technique does not enable determination of the "reflection error correction terms" (reference 3) which are used in electrical reflection calibrations of network analyzers to mathematically remove errors associated with imperfections in the test set hardware. The use of error terms results in a high degree of measurement accuracy with electrical scattering parameters, and the concepts of error correction can be directly applied to fiber optic scattering parameter measurements.
While the above-cited prior art patents are instructive, it is clear that a need remains to provide a microwave fiber optic reflection calibration standard which solves the shortcomings of the prior art. The present invention is intended to satisfy that need.